Pen



FIG. 1 is an exploded right side view; and

FIG. 2 is an exploded left side view; and,

FIG. 3 is an end view thereof.

The broken lines showing the upper part of the barrel in FIGS. 1 and 2 are included for the purpose of showing the environment of the pen and form no part of the claimed design. 

CLAIM The ornamental design for a pen, as shown and described. 